1. Field of the Invention
The present invention relates to a toner image fixing apparatus for fusing and pressing a toner on a recording medium to fix the toner to the recording medium in an image forming system such as a copying machine, a printer, a facsimile machine, etc.
2. Description of the Related Art
One recent toner image fixing apparatus for use in electrophotographic machines is illustrated in FIG. 5 of the accompanying drawings, which is disclosed in Japanese Patent Application (Laid-open) HEI6-318001. As shown in FIG. 5, the toner image fixing apparatus has a fixing roller R1, a heating and tensioning roller R3, an endless fixing belt B trained around the rollers R1, R3, and a pressing roller R2 disposed below and pressed against the fixing roller R1 with the fixing belt B interposed therebetween. When a recording medium D in the form of a sheet with an unfixed toner image carried thereon is fed into the toner image fixing apparatus by a sheet feeder, the recording medium D is reheated by the heating and tensioning roller R3, and then the toner image is fixed to the recording medium D by the fixing belt B in a nipping region between the rollers R1, R2.
Since the recording medium D is preheated, the nipping region may be set to a relatively low temperature. The fixing belt B is of such a small heat capacity that when the recording medium D passes through the nipping region, the temperature of the fixing belt B is quickly lowered to increase the coherent ability of the toner which is separated from the fixing belt B at the outlet of the nipping region, for thereby allowing the toner to be easily separated from the fixing belt B. Even if the fixing belt B is free of oil or coated with a small amount of oil, a clear fixed toner image can be produced on the recording medium D without offsets. The toner image fixing apparatus shown in FIG. 5 is thus capable of solving the problems of toner separation and coil coating, which have not been eliminated by other toner image fixing apparatus using only a heating roller.
The conventional toner image fixing apparatus shown in FIG. 5 will be described in greater detail. The pressing roller R2 is positioned directly beneath the fixing roller R1, and the heating and tensioning roller R3 is disposed upstream of the fixing roller R1 with respect the direction in which the recording medium D is fed into the toner image fixing apparatus along the fixing belt B that is trained around the rollers R1, R3.
The toner image fixing apparatus also has an oil coating roller R4 disposed above an upper run of the fixing belt B. A guide plate G for supporting the recording medium D is disposed below a lower run of the fixing belt B, and a gap between the guide plate G and the lower run of the fixing belt B serves as a preheating passage P for preheating the recording medium D when the recording medium D travels below the heating and tensioning roller R3 toward the nipping region.
The fixing belt B is tensioned to a desired tension level when the heating and tensioning roller R3 is pushed away from the fixing roller R1 by a pressing lever U. The fixing belt B is actuated by the fixing roller R1 which is coupled to an actuator. Since the fixing belt B is appropriately tensioned, it can stably rotate around the rollers R1, R3 without undesirable slippage and sagging.
A heater H is housed in the heating and tensioning roller R3. The heating and tensioning roller R3 is associated with a thermistor S for measuring the temperature of the surface of a core of the heating and tensioning roller R3.
In the conventional toner image fixing apparatus, the gap between the guide plate G and the lower run of the fixing belt B is defined as the preheating passage P for preheating the recording medium D. When the recording medium D carrying an unfixed toner image on its upper surface is transferred along the upper surface of the guide plate G, if the recording medium D is curled, jumps up, or sags due to a speed difference between the toner image fixing apparatus and a preceding toner image transferring apparatus, the recording medium D tends to be partially curved upwardly into contact with the lower run of the fixing belt B. When this happens, the unfixed toner image on the recording medium D is disturbed, failing to produce a desired image on the recording medium D. It has been desired to obviate this drawback.
The above shortcoming is basically caused because of the fixing belt B. One solution, therefore, is to employ a toner image fixing apparatus having no fixing belt B rather than the toner image fixing apparatus shown in FIG. 9. With no fixing belt B used, the heater H has to be housed in the fixing roller R1, and the fixing roller R1 needs to be a metal roller because the outer circumferential surface of the fixing roller R1 should be quickly heated. As a result, if a sufficient nipping width is to be provided in the nipping region between the fixing roller R1 and the pressing roller R2, the pressing roller R2 is required of necessity to be in the form of a resilient roller.
The resilient pressing roller R2 has its outer circumferential surface pressed to a downwardly concave shape by the fixing roller R1 in the nipping region. This downwardly concave nipping region provided by the resilient pressing roller R2, together with the tendency of the recording medium D with the unfixed toner image on its upper surface to stick to the outer circumferential surface of the fixing roller R1, causes the recording medium D to stick easily to the outer circumferential surface of the fixing roller R1. The phenomenon that a recording medium in the form of a sheet sticks to a fixing roller is referred to as a sheet offset. There are demands for improvements to prevent sheet offsets.